An example reductant is an aqueous urea solution optimally blended with 32.5% urea and 67.5% deionized water, and is used in selective catalytic reduction (SCR) exhaust systems to lower NOx exhaust emissions. If the reductant solution becomes diluted from the optimal mixture, the tailpipe NOx emissions will increase. Accordingly, the art of controlling NOx emissions in the exhaust of internal combustion engines will be improved by providing a control system that can adjust the dosing of the reductant if it becomes diluted.